Oldsmobile/Buick Repair: Braking systems, olds achieva, contamination problem

Question
Hi, my girlfriend recently purchased a 1997 Olds Achieva. Ever since she bought it the rear brakes will lock up whenever it sits for more than a few hours. Once she drives it for a short distance (constantly braking) it goes back to normal.  She took it to her local garage and the mechanic bled the entire braking system and filled it with new fluid. He said it might have been caused by contamination. This solved the problem for about a week but it gradually has gotten back to the same problem. The fluid still looks as clean as the day it was put in with no vivible signs of dirt or contamination. What would cause this? If it's not only a contamination problem could it be something related to the ABS system? A bad sensor? I (and everyone else I've talked to) am baffled, I've never ran across anything like this before. I'd greatly appreciate an opinion. Thanks, Roland  

Answer
hello roland
this is a bad abs senser it is on the wheels it self just to see on plug the wires and see if they lock up them now the rear brakes might not have abs depending when the car was made . so then i would check self adj. they could be locking up to pete